1. Field of the Invention
The Present invention relates to a hydrostatic transmission vehicle and a hydrostatic transmission controller.
2. Description of Related Art
There has been conventionally a hydrostatic transmission vehicle which converts an output from an engine by a hydrostatic transmission and travels. The hydrostatic transmission comprises a variable displacement pump driven by an engine, and a variable displacement hydraulic motor which revolves upon receiving a pressure oil from the variable displacement pump. Further, by varying a cam plate angle of the variable displacement pump or the variable displacement hydraulic motor, a displacement of the hydraulic variable transmission can be changed, an engine output which can be absorbed by the hydrostatic transmission can be changed or a vehicle speed of a working vehicle can be changed.
Furthermore, the above-described hydrostatic transmission vehicle include construction machines, and the construction machines includes a vehicle which has a decelerator pedal like a bulldozer so that a vehicle speed can be temporarily reduced by decreasing an engine revolving number by pushing down the decelerator pedal during traveling.
FIG. 7 shows a system configuration of such a conventional vehicle. A fuel injection pump 101 of an engine 100 has a non-illustrated governor lever which adjusts a fuel injection quantity.
This governor lever is connected with a fuel adjustment lever 102 and a decelerator pedal 103 through a push-pull cable and a link mechanism.
An operator of a vehicle can set a revolution number of the engine 100 to a desired revolution number by operating the fuel adjustment lever 102. Moreover, the engine revolution number can be further reduced from the revolution number set by the fuel adjustment lever 102 by stepping on the decelerator pedal 103 so that a vehicle speed can be temporarily decreased. It is to be noted that the decelerator pedal 103 swivels together with the fuel adjustment lever 102 by moving the fuel adjustment lever 102, but a position of the fuel adjustment lever 102 remains unchanged even if the decelerator pedal 103 is pushed down (see, e.g., Japanese Patent Application Laid-open No. 2002-235564).
A potentiometer 104 is provided to the decelerator pedal 103 so that a signal indicative of a position (a swiveling quantity) of the decelerator pedal 103 is transmitted to a controller 105, and an engine revolution sensor 106 is provided to the engine 100 so that a signal indicative of an engine revolution number can be also transmitted to the controller 105.
The controller 105 controls a displacement of a hydrostatic transmission 107 based on signals from the potentiometer 104 and the engine revolution sensor 106.
Here, controlling a displacement of the hydrostatic transmission 107 specifically means controlling a cam plate angle of a variable displacement pump or a variable displacement motor of the hydrostatic transmission 107. That is, changing a displacement of the hydrostatic transmission 107 means varying a torque which can be absorbed by the hydrostatic transmission 107 in torques generated by the engine, and also means varying a ratio of an output revolution number of the hydrostatic transmission 107 to an engine revolution number (i.e., a reduction ratio).
FIG. 8 is a view showing an example of a displacement control of the hydrostatic transmission in the conventional system depicted in FIG. 7.
In FIG. 8, a heavy broken line ET is an engine torque curve representing a relationship between a revolution number of the engine 5 and a torque. Additionally, a heavy solid line HT is an absorption torque curve representing characteristics of a torque which can be absorbed by the hydrostatic transmission 107 in a state where the fuel adjustment lever 102 is set at a maximum position and the decelerator pedal 103 is not pushed down, i.e., the governor lever is moved to a maximum position by the push-pull cable and the engine 100 can revolve at a maximum revolution number (this state will be referred to as a “high-idle” state hereinafter).
More specifically, the absorption torque curve HT is a curve which indicates characteristics representing how a torque with is absorbed by the hydrostatic transmission 107 is varied with respect to an actual change in revolution number of the engine 100 (a horizontal axis in FIG. 8) detected by the engine revolution sensor 106.
As shown in FIG. 8, in the high-idle state, the engine torque curve ET and the absorption torque curve HT are set to cross each other in a revolution number region which is slightly lower than a rated point P0. That is, all of torques generated by the engine 100 are absorbed by the hydrostatic transmission 107 in the vicinity of an engine rated revolution number NH. Further, when a traveling resistance load is increased and an engine revolution number is lowered, a torque to be absorbed by the hydrostatic transmission 107 is rapidly reduced so that an engine stall can be prevented.
With such characteristics, a vehicle can travel by fully using the torque generated by the engine 100 while maintaining a revolution number of the engine 100 in the vicinity of the rated revolution number NH. That is, in the example of a bulldozer mentioned above, a dirt conveying operation can be vigorously and rapidly performed by effectively using an engine output.
When a bulldozer as an example of such a working vehicle performs a dirt conveying operation of pushing dirt as an earthwork, an operator sets a revolution number of the engine 100 to a high idle by operating the fuel adjustment lever 102. As shown in FIG. 8, the engine 100 revolves at the rated revolution number NH and operated at the rated point P0 where a rated torque T0 is generated.
The controller 105 controls the hydrostatic transmission 107 in such a manner that an absorbable torque is set to TK0 which is a value exceeding a torque generated by the engine so that the rated torque T0 can be absorbed. Specifically, a displacement of the variable displacement pump is set to a maximum displacement Q0.
When a load of the dirt with respect to the bulldozer is increased and the revolution number of the engine 100 becomes lower than the high-idle revolution number NH, a revolution number signal of the engine revolution sensor 106 is lowered. Therefore, the controller 105 performs a control of lowering a displacement of the variable displacement pump in accordance with a reduction in the engine revolution number a indicated by a line C in FIG. 8, thereby avoiding an engine stall of the engine 100.
The controller 105 carries out a control of reducing a displacement of the hydrostatic transmission 107 when an engine revolution number becomes lower than the rated revolution number NH in this manner. When a displacement is reduced, a load is decreased and an engine revolution number is increased. Therefore, the hydrostatic transmission 107 eventually maintains a maximum displacement which does not exceed a torque generated by the engine.
Since the engine revolution number is further increased when the load is reduced due to, e.g., an operation of a moldboard by an operator, the controller 105 returns the displacement of the variable displacement pump to the original maximum displacement Q0. Therefore, the bulldozer can always effectively use an output from the engine 100 for the operation.
In case of reducing a speed of the bulldozer, the decelerator pedal 103 is pushed down. Then, the governor lever moves in accordance with a pushing quantity of the decelerator pedal 103, and the revolution number of the engine 100 is reduced. For example, the revolution number is reduced from the rated revolution number NH shown in FIG. 8 to a decelerator revolution number ND.
In this case, since a vehicle speed is not reduced when a displacement of the hydrostatic transmission 107 is left as it is, the controller 105 determines the displacement of the variable displacement pump as a predetermined displacement QD and reduces an absorbable torque to TKD in response to a signal from the potentiometer 104 in order to reduce a vehicle speed.
Again explaining this with reference to FIG. 8, when the decelerator pedal 103 is pushed down in order to temporarily reduce a speed of the vehicle, the governor lever moves in accordance with a pedal pushing quantity, and a fuel injection quantity is restricted. Therefore, the engine revolution number is reduced, and the engine torque curve apparently varies as ET1, ET2 . . . .
Furthermore, at this moment, the controller 105 executes a control of changing the absorption torque curve of the hydrostatic transmission 107 as HT1, HT2 . . . based on a pushing quantity of the decelerator pedal 103 obtained by the potentiometer 104.
As shown in FIG. 8, it is determined that a displacement control pattern of the hydrostatic transmission 107 when the decelerator pedal 103 is pushed down has characteristics of reducing the absorption torque generated by the hydrostatic transmission 107 as a pushing quantity of the decelerator pedal 103 is increased. Therefore, a vehicle speed can be reduced in accordance with a pushing quantity of the decelerator pedal 103.
Meanwhile, in a vehicle such at a construction chine as typified by the above-described bulldozer, a demand for a reduction in noise a working state and a reduction in fuel consumption is increased, and it is often the case that a work is carried out by narrowing down a revolution number of the engine 100 by a manipulation of the fuel adjustment lever 102 in order to effect a partial operation. For example, a work is carried out by reducing a revolution number of the engine 100 to a partial revolution number NP shown in FIG. 8. The engine 100 can generate a partial torque TP with the partial revolution number NP.
When the fuel adjustment lever 102 is operated, however, the decelerator pedal 103 is also moved. Therefore, the controller 105 performs a displacement control (see FIG. 8) of reducing a torque which is absorbed by the hydrostatic transmission 107 based on a detection valve obtained by the potentiometer 104. That is, since a target revolution number of the engine 100 based on a detection value obtained from the potentiometer 104 due to the partial operation is reduced to NP, the controller 105 executes a control by which the absorption torque curve is changed to HT2 so that a displacement of the hydrostatic transmission 107 is reduced. As a result, the absorbable torque of the hydrostatic transmission 107 is reduced to TKP, and hence all of the output torque TP of the engine 100 cannot be absorbed.
A fact that the absorption torque generated by the hydrostatic transmission 107 becomes lower than the torque generated by the engine means that a just small quantity of torque is transmitted to a traveling device irrespective of a fact that the engine torque still has a margin. That is, in the partial operation in this state, engine performances cannot be fully exploited, and the working efficiency is lowered.